Update breakout example's stepping plugin to use children

[krunchington]

Objective

Contributes to #18238
Updates the SteppingPlugin of the breakout example to use the children! macro. Note that in order to test this usage you must use --features bevy_debug_stepping and hit the back-tick key to enable stepping mode to see the proper text spans rendered.

Solution

Updates examples to use the Improved Spawning API merged in #17521

Testing

• Did you test these changes? If so, how?
  • Opened the examples before and after and verified the same behavior was observed. I did this on Ubuntu 24.04.2 LTS using --features wayland.
• Are there any parts that need more testing?
  • Other OS's and features can't hurt, but this is such a small change it shouldn't be a problem.
• How can other people (reviewers) test your changes? Is there anything specific they need to know?
  • Run the examples yourself with and without these changes.
• If relevant, what platforms did you test these changes on, and are there any important ones you can't test?
  • see above

---

Showcase

n/a

Migration Guide

n/a